Epoxides, i.e., organic compounds having one or more terminal or pendant oxirane (epoxy) groups, have been widely used for many years in adhesive compositions. These compounds are commonly cured, or caused to harden, by the addition of a curing or hardening agent. Many epoxy compositions use curing agents that begin to act immediately, or after a short period of time, even at room temperature or lower. These two-part compositions require that the epoxide and the curing agent be stored separately and only mixed immediately before use. A one-part composition has inherent shelf-life stability problems.
In many applications, it is desirable for a mixed two-part composition to be stable for a period of days, weeks, or months so that large batches can be prepared to be used as needed. Various attempts have been made to prolong "pot life" prior to cure of curable epoxy compositions. This allows the epoxide and curing agent to be mixed and stored before use as a one-part that is curable upon heating. Generally, it has been found that any increase in pot-life of epoxy compositions results in a corresponding sacrifice of cure speed. Additionally, achieving complete reaction, i.e., where all the epoxide functional groups are consumed, may require longer cure times and/or higher cure temperatures.
Various publications disclosing the preparation and/or use of curing agents for epoxy resins and/or other curable compositions are discussed below.
European Patent Application 0 511 405A1 teaches the use of onium salts having a nucleophilic pair anion, such as halogenide, perchlorate, alkylsulfate, arylsulfonate, as stabilizers for the polymerization of cationically polymerizable organic compositions, catalyzed by (n6-arene)(n.sup.5 -cyclopentadienyl)iron(+1) salt cationic photoinitiators. The pair anion of the onium salt attacks a growing terminal of the polymerization reaction during the curing of the composition leading to rapid termination.
U.S. Pat. Nos. 5,130,406 and 5,179,179 disclose cationic initiators containing oxygenated sigma-donor ligands. Such initiators suffer from their high hygroscopicity (moisture sensitivity); additionally, these initiators provide very little work time because they must be mixed with the epoxide at low temperatures.
Iron-arene salt cationic photoinitiators have been shown to effect the polymerization of pyrrole in the presence of oxygen. (Rabek, et al. Polymer 1992, 33, 4838-4844.) This polymerization proceeds through proton and hydrogen atom abstraction catalyzed by Fe(III). The polymerization was completely inhibited in the presence of 1,10-phenanthroline.
U.S. Pat. No. 4,920,182 discloses the use of a photoactivated bis-arene Fe complex that initiates copolymerization of epoxy- and carboxyl-terminated polyesters after exposure to light and subsequent heating. There is, however, no indication of the degree of either pot-life before, or latency, after, photoactivation as well as the speed of cure and its relation to pot-life and latency.
U.S. Pat. No. 4,846,905 describes the use of an acid/amine complex as a catalyst for copolymerizing epoxides and polyols. While excellent utility as a structural adhesive for bonding oily galvanized steel is shown, this particular catalyst system sacrifices pot-life to achieve rapid cure at low to moderate temperatures.
U.S. Pat. No. 4,751,138 teaches a coated abrasive article prepared by polymerizing a combination of epoxy and acrylate monomers using a combination of photoinitiators which can be cationic organometallic salts, onium salts, or thermally active Lewis acids such as antimony pentafluoride. Stabilization of the Lewis acids by the addition of amines is taught.
Australian Patent Application 38551/85 describes a hardenable composition consisting of a) a material polymerizable by cationic or free radical polymerization b) an iron (II)-.eta..sup.6 -benzene-.eta..sup.5 -cyclopentadienyl complex salt, c) a sensitizer, and with certain monomers d) an electron acceptor. The electron acceptors are preferably an organic hydroperoxide, an organic peracid or a quinone.
U.S. Pat. No. 5,073,476 teaches the combination of iron(II) aromatic complexes mixed with electron acceptors for the polymerization by irradiation of organic materials which can be polymerized cationically and/or by free radicals. Nonspecific reference is made to methods to increase the capacity of the compositions to be stored in the dark by the addition of weak organic bases which are broadly stated to be nitriles, amides, lactones or urea derivatives.
U.S. Pat. No. 5,089,536 disclose curable compositions comprising an ionic salt of an organometallic complex cation and a cationically curable monomer. Novel compounds disclosed therein are claimed in U.S. Pat. No. 5,191,101. There is no teaching as to how to increase the shelf life or pot-life of a cationically curable composition.
Canadian Patent Application 2,040,010 relates to epoxy resin compositions comprising iron-arene complexes and specific primary amines or bipyridines as inhibitors. The epoxy containing compositions require light activation to cure.
U.S. Pat. No. 4,503,211 teaches that a latently curable epoxy resin composition including a novel curing agent comprising the liquid salt of a substituted pentafluoroantimonic acid and an aromatic amine has long pot life yet cures rapidly when heated.
U.S. Pat. No. 5,047,376 teaches the activation of organic cationically polymerizable materials through the use of a dispersion or a solution of an iron/arene salt and a polycarboxylic acid, an anhydride based on a polycarboxylic acid, or a polyisocyanate, by heating or with actinic irradiation. They teach increased shelf life using these compositions.
A conductive metal-containing adhesive is disclosed in Canadian Patent No. 1,277,070. Organic solvents useful to make coatable compositions are disclosed and include certain sulfoxides, sulfones, carboxylates, and lactones.